The macrophage, an important component of the immune system, in large part functions to detect and damage foreign cells. Since macrophages interact with other target cells primarily through surface receptors which recognize IgG-\and C3-coated cells, it is likely that such receptors are similarly important in tumor cell recognition. Macrophage-lymphoblastoid cell interaction is being examined as a model of macrophage-tumor cell interaction. A spontaneous interaction has been observed and characterized. The capacity of complement to augment this interaction is being studied, using tissue as well as circulating macrophages. In addition, the effect of the pharmacological agents, corticosteroids, levamisole, and N-formyl peptides are being studied. Delineation of the mechanisms involved in macrophage recognition of lymphoblastoid cells should provide insight into new approaches for harnessing the macrophage's ability to detect tumor cells and modify their viability. The role of macrophage subsets and the expression of their Fc (IgG) and C3 receptors are also being examined as well as the role of the macrophage in the regulation of normal and disordered immunoglobulin production in multiple myeloma. (MB)